Electron lens having means for correcting astigmatism



May 24, 1966 G. F. REMPFER 3,253,144

ELECTRON LENS HAVING MEANS FOR CORRECTING ASTIGMATISM Filed May 27, 19632 Sheets-Sheet l e. I 4e GE/PTRUDE E REMPFER /NVENTO/. BY

BUC/(HORN, BLORE, KLROU/.ST 8 SPAR/(MAN ATTORNEYS May 24, 1956 G. F.REMPFER 3,253,144

ELECTRON LENS HAVING MEANS FOR CORRECTING ASTIGMATISM Filed May 27, 19632 Sheets-Sheet 2 GERTRUDE REM/FER /NVENTOR BUC/(HORN, BLORE, KLAHOU/ST 8SPAR/(MAN A 7' TOR/VE YS United States Patent O M' 3,253,144 ELECTRONLENS HAVING MEANS FOR CORRECTING ASTIGMATISM Gertrude F. Rempfer, ForestGrove, Oreg., assignor to Tektronix, Inc., Beaverton, Orcg., acorporation of Oregon Filed May 27, 1963, Ser. No. 283,424 6 Claims.(Cl. Z50-49.5)

The subject matter of the present invention relates generally toelectron lenses of the electrostatic type, and in particular to lenseswhich are compensated to correct for image aberrations, such asastigmatism, and to methods of reducing such astigmatism.

The electron lens of the present invention is particularly useful whenemployed as the object lens of an electron microscope or in otherelectron optics systems. All uncompensated electron lenses suffer fromthe image aberration defect known as astigmatism due to the fact thatthey are not perfectly symmetrical about the lens axis so that theelectrostatic focusing fields produced by the lens are not symmetrical.As a result of this axial asymmetry, the images produced -by the lensare impaired in stigmatic quality due to a variation of the focaldistance of the lens for different electron paths at various azimuthsabout the lens axis. The term focal distance is used throughout thepresent application to mean the distance from the lens center to a pointon the lens axis where the image is in focus. Thus, it is apparent thatan uncompensated electron lens suffering from the astigmatism referredto above will have a plurality of focal distances, rather than one suchfocal distance, including a maximum focal distance and a minimum focaldistance. The compensated lens of the present invention is corrected forastigmatism by providing two electrodes with pairs of axial asymmetriesor discontinuities which are positioned to introduce a correction eldhaving a maximum correction axis that is employed to move either themaximum focal distance or the minimum focal distance into registrationwith the correct focal distance for the lens. This tends to equalize thefocal distance at all azimuths and thus, corrects the lens forastigmatism so that the resolution of such lens is increased by asubstantial amount.

The electron lens of the present invention is an improvement over myprevious electron lens, disclosed in U.S. Patent 2,536,878 which issuedJanuary 2, 1951, to Gertrude M. Fleming and is entitled Electron Lens.The present electron lens has several advantages over conventionalelectron lenses including the one described `in the above-mentionedpatent. The primary advantage of the present lens is the employment ofan extremely simple structure to correct such lens for astigmatism whichis less expensive to manufacture and easier to adjust than previouscompensation devices. Another advantage of the electron lens of thepresent invention is that it enables a simpler and faster method ofadjusting the lens to reduce astigmatism. In addition, the increase inresolution obtained by the compensated lens of the present invention iscomparable with that obtained by conventional lenses employing much morecomplicated and expensive compensation devices and methods ofadjustment.

One of the greatest causes of defects in image formation by an electronlens is the axial asymmetry due to the irregular shape of the aperturesof the electrodes of such lens through which the electron Ibeam passes.For proper operation of the lens, the electron beam apertures should beperfectly circular in planes perpendicular to the common axis of thelens. However, it is impossible to make apertures of a perfectlycircular shape and even 3,253,144 Patented May 24, 1966 ICC very smalleccentricities in the apertures produce sui-licient asymmetry in thefocusing fields to defocus the highly magnified images obtained inelectron microscopes. In a three element electron lens, the diameter ofthe aperture in the central electrode and the thickness of such centralelectrode greatly affect the focal properties of such lens, While thedistance between the two outer electrodes of the lens also controls suchfocal properties to a lesser extent. The lens of the present inventioncompensates for astigmatism by effectively varying the distance as afunction of azimuth 4between the outer electrodes. A line adjustment canbe made in this manner which is much simpler than that made by variationof the aperture diameter or thickness lof the central electrode.

Briefly, this compensation is accomplished by forming a pair ofdiscontinuities on the inner side surface of each of the entrance andexist electrodes on opposite sides of the beam apertures of suchelectrodes adjacent the periphery of such apertures. If both of thesepairs of discontinuities are projections, the entrance and exitelectrodes are positioned in alignment with the astigmatism azimuth ofminimum focal distance which may be found by conventional methodsindicated in U.S. Patent 2,536,- 878 referred to above. If the amount ofcorrection is too great and causes over-correction, it is reduced to theproper amount by rotating the exit and entrance electrodes with respectto each other through an angle less than while maintaining thecorrection axis in alignment with the astigmatism azimuth.

It is therefore one object of the present invention to provide animproved electron lens of the electrostatic type.

Another object of the invention is to provide an improved electron lenswhich is corrected for astigmatism.

A further object of the present invention is to provide an improvedelectron lens of a simple and inexpensive structure which can be quicklyand easily adjusted to correct for image aberration caused by axialasymmetry.

An additional object of the invention is to provide an improved methodof reducing astigmatism in electron lenses.

Still another object of the invention is to provide an improved electronlens for `an electron microscope in which the entrance and existelectrodes employed in such lens are each provided with a pair ofdiscontinuities on their inner side surfaces on opposite sides of thebeam apertures through such electrodes to reduce the astigmatism of suchlens and to increase the resolution of such microscope.

Other objects and advantages of the present invention will be apparentfrom the following detailed description of certain preferred embodimentsthereof and from the attached drawings of which:

FIG. l is a plane view of the exit end of an electron lense made inaccordance with the present invention,

FIG. 2 is a horizontal sectional view taken along the line 2-2 of FIG.l,

FIG. 3 is an enlarged view of a portion of FIG. 2 showing one embodimentof the compensation discontinuities of the present invention,

FIG. 4 is an enlarged View of a portion of FIG, 2 showing anotherembodiment of the compensation discontinuities of the present invention,

FIGS. 5A, 5B and 5C show different positions of the discontinuities ofFIG. 3 corresponding to the zero, intermediate and maximum amounts ofcorrection, and

FIGS. 6A, 6B and 6C show the zero, intermediate and maximum correctionpositions, respectively, of entrance and exit electrodes employing athird embodiment of the compensation discontinuities of the presentinvention.

As shown in FIGS. 1 and 2, the electron lens includes a hollowcylindrical lens cell or mount which holds an entrance electrode 12, acentral electrode 14, and an exit electrode 16 in a spaced, coaxialrelationship. Each of these electrodes is in the form of substantiallyaxially symmetrical diaphragm having a circular center portion ofreduced thickness through which a substantially circular beam apertureextends. The three electrodes 12, 14 and 16 are coaxially mounted on acommon axis 17 passing through the center of their electron beamapertures so that an electron beam enters the lens through entranceelectrode 12 and leaves through exit electrode 16. These electrodes areheld in spaced relationship by first and second spacer rings 18 and 20positioned between the electrodes. The spacer rings 18 and 2l)v may bemade of alumina ceramic material or other suitable electrical insulatorsso that the rst spacer ring electrically insulates the central electrode14 from the entrance electrode 12, while -the second spacer ringinsulates the exit electrode 16 from such central electrode. After theelectrodes and spacer rings have been properly assembled within the lensmount 10 by inserting them through the rear en-d of such mount until thefront surface of the entrance electrode 12 engages an annular inwardlyextending shoulder portion 22 adjacent the front end of such mount, thisassembly is clamped in the mount by a retaining ring 24 which is screwthreaded into the rear end of the mount to prevent movement of suchelectrodes. It should be noted that the central electrode 14 is held inaxial alignment with the entrance and exit electrodes by means of anannular forwardly extending flange portion 26 on the second spacer ring20. This spacer ring is held in axial alignment with the exit electrode16 by means of an annular forwardly extending flange portion 28 on theperiphery of such exit electrode.

The central electrode 14 is connected to a source of high negative D.C.voltage by means of a lead rod 30 having a spring biased plunger 32mounted in one end thereof for resilient engagement with the outer edgeof the central electrode through an opening in the side of the lensmount 10 and a notch in the flange portion 26 of spacer ring 20. Thehigh voltage lead rod 30 is provided with `a pair of elongated slots 34through the opposite sides of such rod into a cylindrical cavity formedin one end of such rod. The plunger 32 is provided with a pair of stops36 which project into the slots 34 to limit axial and rotationalmovement of the plunger within the lead member. The lead rod is suitablymounted so that the plunger extends substantially perpendicular to thelens mount 10. A coil spring 38 positioned within the cavity at one endof the lead rod urges the plunger into electrical contact with the outeredge of the central electrode. The entrance and exit electrodes areelectrically contacted to each other through the metal lens mount 10which is grounded so that `the central electrode is negative withrespect to these two outer electrodes and an electrostatic focusingfield is produced by said lens. i

It should be noted that an opening 40 is provided in the side of thelens mount 10 through the shoulder portion 22 to allow the insertion ofa specimen holder into the front end of the lens mount so that suchspecimen is positioned directly in front of the beam aperture in theentrance electrode 12. In addition, a shield plate 42 of high magneticpermeability metal is provided over the front end of the lens mount withthe enlarged aperture of such plate in alignment with the lens `axis 17.The shield plate is held in place by a retaining ring 44 similar toretaining ring 24, which is screw threaded into the front end of thelens mount to clamp such shield plate between such ring and the shoulderportion 22. This shield plate-'helps prevent external electrical ormagnetic rields from distorting the focusing field produced inside thelens holder.

In order to compensate the electron lens for astigmatism, the entranceelectrode 12 may be provided with a pair of axial asymmetries ordiscontinuities in the form of projections 46 on the inner surface ofsuch electrode, as shown in FIG. 3. These projections are positioned onopposite sides of the beam aperture of electrode 12 adjacent theperiphery of such aperture and located on a common diameter thereof sothat they are approximately degrees apart. The projections may be formedby metal deposits which are coated on the entrance electrode bysputtering metal through a mask with openings cut in the shape of theprojections. The shape of these deposited projections is not criticaland they may belabrupt or gradual discontinuities. However, it has beenfound that for practical reasons, it is desirable to make theprojections identical in height and area so that they have substantiallyequal compensation effects on the focusing field. This also avoidsmisalignment of the electron beam. When similar shaped projections areemployed they are positioned substantially the same distance from thelens axis 17. A similar pair of projections 50 may be provided on theinner surface of the exit electrode 16 so that they are positioned onthe opposite sides of the beam aperture in such electrode on a diameterthereof separated by about 180. Each of these projections may extendapproximately .001 inch above the surface of the electrodes when thediameter of the aperture in the central electrode is on the order of .linch and the distances between the central electrode and the outerelectrode are about .l inch.

FIG. 4 shows another embodiment of the present invention similar to FIG.3 in which the projections 46 and 50 have been replaced by indentations52 and 54 on electrodes 12 and 16', respectively. The indentations arealso vaxial asymmetries or discontinuities which may be employedv tocompensate for any astigmatism in the lens. These indentations may alsobe formed substantially identical with a depth of approximately .001inch, in any suitable manner such as by chemically etching through amask, by mechanical abrasion or cutting.

The rotational position of projections 46 and 50 about the common axis17 determines the amount of correction introduced by such projections,as shown in FIGS. 5A, 5B and 5C. In FIG. 5A, the diameter lines 56 and5S of projections 46 and 50, respectively, are positioned at rightangles to each other. In this position the projections produce noeffective correction on a circular cross section 57 electron beam shownenlarged for clarity, because they compensate for each other. However,in FIG. 5B, the diameter lines 56 and 58 are displaced by an angle of 45and in this intermediate position the projections produce a maximumcorrection plane whose axis 59 lies approximately midway between thediameter lines of projections 416 'and 50. This results in expansion ofthe beam cross section 57 along axis 59. If the exit and entranceelectrodes are positioned so that the diameter lines are in alignment,-as shown in FIG. 5C, the projections produce a maximum amount ofcorrection which expands the beam cross section 57 even further in theplane determined by the diameters of the pairs -of projections alongcorrection axis 59.

In order to correct for astigmatism in the electron lens of FIGS. 2 and3, the uncompensated entrance and exit electrodes are positioned withinthe lens mount on opposite sides of the 4central electrode "14 and theresulting lens structure is tested in a conventional manner to determinethe astigmatism azimuth of minimum focal distance, for example, in themanner recited in U.S. Patent No. 2,536,878. Next, the exit and entranceelectrodes are removed from the lens mount and provided with thecorrecting projections 46 and 50. Then these compensated entrance andexit electrodes 12 and 16 are inserted back into the lens mount 'andpositioned so that they exert the maximum correction of FIG. 5C and arelocated so that the axis 59 of maximum correction is in alignment withthe astigmatism azimuth of minimum focal distance. The lens is againtested for astigmatism. Iff it is found that the lens has beenovercorrected, the retaining ring 24 is loosened slightly and the exitelectrode 16 and the entrance electrode 12 are rotated with respect toeach other by suitable tools inserted into two pairs of notches 60 and61. These notches 60 and 61 are provided in the outer side surfaces ofelectrodes 12 and 16, respectively, out of the focusing field to enablerotation of the diameters of the pairs of projections 46 and 50 intosome intermediate position similar to that shown in FIG. 5B. Thisreducesthe amount of correction and may be repeated until such correctionexactly compensates for the astigmatism of t-he lens if the axis ofcorrection is maint-ained in alignment with the astigmatism azimuth ofminimum focal distance during rotation of the electrodes. After thiscondition has been obtained, the retaining ring 24 is tightened t-oclamp the electrodes in their correct angular positions. The centerelectrode 14 is held in a fixed position during rotation by pressing asuitable tool against the periphery Vof such electrode in place of thelead rod and plunger. It may also be desirable to provide a referencemark 63 on the side of the lens mount 10 and a scale 64 on the outerside surfaces of each of the electrodes 12 and 16 in order to aid in theadjustment of such electrodes and to enable quick assembly of the lensafter it has been so adjusted.

The entrance and exit electrodes 12 and 16 of FIG. 4 may be adjustedwith respect to the center electrodes 14 in a similar manner to thatalready described with reference to FIG. 3. However, since thecorrecting discontinuities employed in these electrodes are indentationsrather than projections, the axis of maximum correction must bepositioned in alignmentwith the astigmatism azimuth of maximum focaldistance rather than the azimuth of minimum focal distance. Other thanthis, the method steps are the same and the position of the maximumamount of correction is obtained with the diameter of the indentations512 and 54 in alignment as shown in FIG. 5C. It should be noted that thecorrection produced by the indentation is opposite to that produced bythe projections so that the beam cross section 57 is compressed in themaxim-um correction plane 59.

IIowever,it is also possible to employ a combination of differentdiscontinuities including a pair orf indentations and a pair ofprojections. If a pair of indentations 65 is provided on the entranceelectrode 112" and a pair of projections 66 is provided on the exitelectrode .16 these discontinuities will compensate for each other toproduce no ycorrection when they are in alignment as shown in FIG. 6A.Thus, if the height of the projection and the depth of the indentationare equal, then in this position the distance between the two outerelectrodes is the same when measured from a projection to an indentationas it -is when measured between the other surface portions oftheelectrodes. However, if the pairs of indentations and projections 65 and66 are rotated out of alignment into the intermediate position of FIG.61B, some correction will result due to the change in the focusingyfield caused by the vaniation in the distance between the two outerelectrodes. An axis of maximum correction 67 may be determined for theastigmatism azimuth of maximum focal distance by adding a vector 68extending perpendicular to the diameter of the projections 66, to a|vector 69 corresponding to the axis of indentations 65. In addition,another axis of maximum correction 70 may be determined for theIastigmatism azimuth of minimum focal distance in a similar manner. `Itshould be noted that these two axes 67 and 70 are at right angles toeach other. For a maximum amount of correction, the diameters of theindentations 615 and the projections 66 are positioned perpendicular toeach other, as shown in FIG.

6C. This produces the pair of maximum correction axes 70 and 67 whichintersect each other at right angles in alignment with the dia-meters ofthe projections and indentations, respectively. Therefore, either ofthese correcti-on axes of FIG. 6C may be pl-aced in alignment with theproper astigmatism azimuth and the pairs of discontinuities are thenrotated toward each other in order to reduce the amount ofcorrectionwhile maintaining the correction axis in alignment with the astigmatismazimuth in the manner of FIG. 5.

It will be obvious to those having ordinary skill in the art thatvarious changes may be made in the details of the above-describedpreferred embodiments of the present invention Without departing fromthe spirit of the invention. Therefore, the scope of the presentinvention should only be determined by the following claims.

I claim:

1. An electrostatic electron lens, comprising:

a plurality of electrically conductive electrodes each having asubstantially circular aperture for the passage of electrons throughsaid electrodes;

means for ycoaxially mounting lsaid electrodes so that a first electrodeis positioned between and is in spaced insulated relationship from asecond electrode and a third electrode;

a first pair of dimensional discontinuities provided on the inner sidesurface of said second electrode nearest said fir-st electrode, s-aidfirst pair of discontinuities being permanently fixed relative to thesecond electrode and to each other and positioned adjacent the peripheryof the aperture of said second electrode on opposite sides of theaperture along a diameter line of said aperture; and

a second pair of dimensional discontinuities provided on the inner sidesurface of said third electrode nearest said first electrode, saidsecond pair of discontinuities being permanently fixed relative to thethird electrode and to each other and positioned adjacent the peripheryof the center aperture of :said third electrode on opposite sides of theaperture along a diameter line of said aperture.

2. An electrostatic electron lens, comprising:

a plurality of electrically conductive electrodes each having asubstantially circular aperture for the passage of electrons throughsaid electrodes;

means for coaxially mounting said electrodes so that a central electrodeis positioned between and is in spaced insulated relationship from anentrance electrode and an exit electrode;

a first pair of compensation discontinuities provided on the inner sidesur-face of said entrance electrode nearest said central electrode, saidfirst pair of discontinuities being permanently fixed relative to theentrance electrode and to each other positioned adjacent the peripheryof the `aperture of said entrance electrode on opposite sides of theaperture along a diameter line thereof;

a second pair of compensation discontinuities provided on the inner sidesurface Iof said exit electrode nearest said central electrode, saidsecond pair of discontinuities being permanently fixed relative to theexit electrode and to each other and positioned adjacent the peripheryof the aperture of said exit electrode on opposite sides of the aperturealong a diameter line thereof; and

means for rotating said entrance electrode and said exit electrode withrespect to each other -about their common axis in order to change theangular displacement of said first and second pairs of discontinuitiesso that astigmatism of the lens is reduced.

3. An electrostatic electron lens, comprising:

a plurality of substantially symmetrical, electrically conductiveelectrodes each having a substantially circular center aperture whoseiaxis corresponds to the axis of symmetry of said electrodes for thepassage of electrons through said electrodes;

means for coaxially mounting said electrodes so that a central electrodeis positioned between and is in spaced insulated relationship from anentrance electrode yand an exit electrode;

a rst pair of compensation projections -of substantially the sameeffective size provided on the inner side :surface of said entranceelectrode nearest said central electrode, sai-d first pair ofprojections 'being fixedly positioned adjacent the periphery of thecenter aperture of said entrance electrode on opposite sides of theaperture along a diameter line thereof at substantially equal distancesfrom the axis of said entrance electrode;

a second pair of compensation projections of substantially the sameeffective size provided on the inner side surface of said exit electrodenearest said central electrode, said Second pair of projections beingfixedly positioned adjacent the periphery of the center aperture of saidexit electrode on opposite sides of the aperture along a diameter linethereof at substantially equal distances from the laxis of said exitelectrode; and means for rotating said entrance electrode and said exitelectrode with respect to each other about their common axis in order tochange the angular displacement of said first and second pairs ofprojections, and for enabling relative rotation between said centralelectrode and said exit and entrance electrodes about the axis, so thatastigmatism of the lens is reduced.

4. An electrostatic electron lens, comprising:

a plurality of substantially symmetrical, electrically conductiveelectrodes earch having a substantially circular center aperture whoseaxis correspon-ds to the axis of symmetry of said electrodes for thepassage of electrons through said electrodes;

means for coaxially mounting said electrodes so that a central electrodeis positioned between and is in spaced insulated relationship `from anentrance electrode and an exit electrode;

a first pair of compensation indentations provided on the inner sidesurface of said entrance electrode nearest said central electrode, saidfirst pair of indentations being fixedly positioned adjacent theperiphery of the center aperture of said entrance 'electrode on oppositesides of the aperture along a diameter line thereof at substantiallyequal distances from the axis of said entrance electrode;

a second pair of compensation indentations provided on the inner sidesurface of said exit electrode nearest said central electrode, saidsecond pair of indentations being xedly positioned adjacent theperiphery of the `center aperture of said exit electrode on oppositesides of the aperture along a diameter line thereof at substantiallyequal distances from the axis of said exit electrode; and

means for rotating said entrance electrode and said ex-it electrode withrespect to each other about their common axis in order to change theangular displacement of said first and second pairs of indentations, andfor enabling relative rotation between said central electron and saidexit and entrance electrodes about the axis, so that astigmatism of thelens is reduced.

5. An electrostatic lens comprising:

at least three electrodes each having an aperture for 8 the transmissionof an electron beam through said electrodes;

means for mounting said three electrodes in spaced insulatedrelationship with their apertures in axial lalignment to provide firstand second outer electrodes and an intermediate electrode between saidouter electrodes;

a plurality of first discontinuities provided on the inner surface ofsaid first outer electrode between said first outer electrode and saidintermediate electrode, said first discontinuities being fixed relationto said first outer electrode and to each other and spaced about theperiphery of the aperture of said first outer elec-I trode; andplurality of second discontinuities equal in number to said firstdiscontinuities provided on the inner surface of said second outerelectrode between said second outer electrode and said intermediateelectrode, said second discontinuities being fixed relative to saidsecond outer electrode and to each other and spaced about the peripheryof the aperture of said second outer electrode.

6. An electrostatic lens comprising:

at least three electrodes each having an aperture for the transmissionof an electron beam through said electrodes;

means for mounting said three electrodes in spaced insulatedrelationship with their apertures in axial alignment to provide firstand second outer electrodes and an intermediate electrode -between saidouter electrodes;

a plurality of first dimensional discontinuities provided on the innersurface of said first outer electrode between said first outerelect-rode and said intermediate electrode, said first discontinuitiesbeing xed rela tive to said first outer electrode and to each other anduniformly spaced about the periphery of the aperture of said first outerelectrode;

a plurality of second dimensional discontinuities equal in number tosaid first discontinuities provided on the inner surface of said secondouter electrode 'between said second outer electrode and saidintermediate electrode, said second discontinuities being fixed relativeto said lsecond outer electrode and to each other and uniformly spacedabout t-he periphery of the aperture of said second outer electrode; and

means for rotating said outer electrodes with respect to each otherabout the common axis of their aperture-s to change the angulardisplacement of said first discontinuities and said seconddiscontinuities.

References Cited by the Examiner UNITED STATES PATENTS 2,536,878 l/l951Fleming 2SC-49.5 X 2,754,443 7/1956 Asmus Z50-49.5 X 2,910,603 10/1959Van Dorsten et al. Z50-49.5

RALPH G. NILSON, Primary Examiner.

HENRY S. MILLER, ANTHONY L. BIRCH,

Assistant Examiners. v

1. AN ELECTROSTATIC ELECTRON LENS, COMPRISING: A PLURALITY OFELECTRICALLY CONDUCTIVE ELECTRODES EACH HAVING A SUBSTANTIALLY CIRCULARAPERTURE FOR THE PASSAGE OF ELECTRONS THROUGH SAID ELECTRODES; MEANS FORCOAXIALLY MOUNTING SAID ELECTRODES SO THAT A FIRST ELECTRODE ISPOSITIONED BETWEEN AND IS IN SPACED INSULATED RELATIONSHIP FROM A SECONDELECTRODE AND A THIRD ELECTRODE; A FIRST PAIR OF DIMENSIONALDISCONTINUITIES PROVIDED ON THE INNER SIDE SURFACE OF SAID SECONDELECTRODE NEAREST SAID FIRST ELECTRODE, SAID FIRST PAIR OFDISCONTINUITIES BEING PERMANENTLY FIXED RELATIVE TO THE SECOND ELECTRODEAND TO EACH OTHER AND POSITIONED ADJACENT THE PERIPHERY OF THE APERTUREOF SAID SECOND ELECTRODE ON OPPOSITE SIDES OF THE APERTURE ALONG ADIAMETER LINE OF SAID APERTURE; AND A SECOND PAIR OF DIMENSIONALDISCONTINUITIES PROVIDED ON THE INNER SIDE SURFACE OF SAID THIRDELECTRODE NEAREST SAID FIRST ELECTRODE, SAID SECOND PAIR OFDISCONTINUITIES BEING PERMANENTLY FIXED RELATIVE TO THE THIRD ELECTRODEAND TO EACH OTHER AND POSITIONED ADJACENT THE PERIPHERY OF THE CENTERAPERUTURE OF SAID THIRD ELECTRODE ON OPPOSITE SIDES OF THE APERTUREALONG A DIAMETER LINE OF SAID APERTURE.